1. Field of the Invention
This invention relates generally to an apparatus, such as an optical disc player, in which a light beam is directed against a disc or other record medium surface for optically reading information recorded in a track on such surface, and more particularly is directed to a device for movably supporting an objective lens in such apparatus.
2. Description of the Prior Art
It is known, for example, as disclosed in U.S. Pat. No. 4,135,083, to provide an optical disc player with a focusing servo by which the light beam from a laser light source is correctly focused on the disc surface through an objective lens, and also with a tracking servo by which the point of impingement of the focused light beam on the disc surface is made to coincide with the track being scanned or read. Further, in the above identified U.S. patent, and particularly in FIGS. 7-10 thereof, it is disclosed to support the objective lens by first and second sets of springs so that the objective lens can be moved, in response to electromagnetically applied forces, both in the axial direction for correcting the focusing errors, and in a direction extending radially in respect to the track being scanned for correcting tracking errors. However, the use of a number of springs to support the objective lens, as aforesaid, gives rise to problems, in that the interrelationships of the resonances of the several springs have to be considered. Furthermore, the mass of the movable member which includes the objective lens is relatively large. Therefore, in order for the movable member to have the proper kinetic characteristics for responding to tracking and focusing error signals, it is necessary for either the driving current or the number of coil turns in its associated electro-magnetic driving means to be relatively large.
It is also known to provide an apparatus of the described type with an objective lens which is mounted for movements to correct focusing and tracking errors without resort to mounting springs for permitting such movements. For example, as shown diagramatically on FIG. 1, in such known apparatus, a yoke plate 1 slidably supports a shaft 2 to permit both axial displacements of the shaft and angular displacements of the latter about its longitudinal axis. An objective lens is supported in a mount 3 which is carried by the free end of an arm 4 extending radially from shaft 2. A voice coil 5 is wound on a bobbin secured to the lower end of shaft 2 and intersects a magnetic field generated by a magnetic circuit which includes yoke plate 1 and an annular magnet 7 secured thereto. When a focusing error signal is applied to coil 5, shaft 2 is displaced axially to similarly move objective lens mount 3. Another coil 8 is mounted on the upper portion of shaft 2 and cooperates with a magnetic circuit including an additional magnet 9 so that, when a tracking error signal is supplied to coil 8, shaft 2 is angularly displaced about its longitudinal axis to correspondingly angularly displace arm 4 and thereby move objective lens mount 3 in an arcuate path which is generally normal to the paper in FIG. 1. As is usual, in the apparatus of FIG. 1, a laser light source S generates a light beam L reflected by a beam splitter or semi-reflecting mirror M so as to travel along a path which is normal to the surface of an optical record disc R and in which the objective lens in mount 3 is interposed for focusing the light beam substantially at the surface of disc R. The focused light beam, upon being reflected from the surface of disc R is returned through the objective lens and now passes through beam splitter M so as to impinge on a photo-sensitive detector or transducer T. The reflected light beam is, of course, modulated by an information signal recorded in a track on the surface of disc R and which is scanned by the focused light beam so that the output of transducer T is similarly modulated to reproduce or playback the recorded signal. Since the axis of shaft 2 is normal to the surface of disc R, axially displacements of shaft 2, as indicated by the arrow a, in response to the supplying of current to coil 5, move the objective lens in mount 3 toward and away from the record disc surface for varying the focus of the light beam. Further, the discretion of arm 4 is selected to be substantially tangential in respect to substantially circular tracks in which the information signals are recorded on the surface of disc R. Therefore, angular displacements of shaft 2, as indicated by the arrow b, in response to the supplying of a tracking error signal to coil 8 cause the objective lens in mount 3 to move substantially transversely in respect to the track being scanned for correcting a tracking error.
However, the known arrangement described above with reference to FIG. 1 is disadvantageous in that it is inherently large and massive, as well as being complicated to produce and assemble.